Auxiliary power units (APUs) are used in the aircraft industry to provide auxiliary power to onboard systems. APU systems may include turbine, fuel cell, and hybrid systems. Turbine-driven APUs burn jet fuel to drive a turbine, which in turn drives a generator to produce electrical energy. Fuel cells chemically convert fuel into electrical energy. Hybrid APUs utilize both turbine and fuel cell methods in its production of power. Each type of APU may be used to power onboard systems while the aircraft is on the ground, as well as when the aircraft is in flight.
Disadvantages of turbine APUs, however, may include poor fuel efficiency and appreciable pollution to an airport environment. In the case of turbine APUs, the burning of fuel to drive the turbines leaves some jet fuel uncombusted and produces oxides of nitrogen (NOx) emissions. Fuel cell APUs offer lower emissions than turbine-driven APUs, and are more fuel efficient, but are heavy and tend to loose efficiency at high altitudes. Hybrid fuel cell APUs generally improve upon the efficiency of conventional turbine and fuel cell APUs and are lighter weight than pure fuel cell APUs, but provide only electrical power suitable for certain aircraft and not for others, including those that require compressed air to drive an environmental control system.
An environmental control system (ECS) provides an environment with conditioned cool or warm compressed air. The ECS is typically pneumatically powered by hot compressed air that is bled off the aircraft engine's compressor or turbine-powered APU compressor. Alternately, the ECS can incorporate motors that are electrically powered by the aircraft engine generator or APU generator However, an ECS will penalize engine performance because its energy source is derived directly from the main engine or APU, capturing a significant amount of power.
Thus, conventional ECS packs that are pneumatically driven by the airplane engine or APU tend to be light weight, but are relatively inefficient. An ECS pack that is electrically driven by the airplane engine generator or a (hybrid) Fuel Cell APU is more efficient but is heavy.